How to Change Inventory Unit ?

Dear All AX Gurus!

Scenario:

We are using Dynamics AX 2009, we have an item which have Purchase Unit = m3 (Cubic Meter) and Inventory Unit = LN (Length), and the said item has been already received through PO (Purchase Order), it means this item has transaction against it.

Desired Solution:

We want to change the Inventory Unit from LN(Length) to m3 (Cubic Meter), as the transaction exist so Dynamics AX 2009 is not allowing to change the Inventory Unit, I know this is the default behavior of Dynamics Ax 2009.

Could anyone tell me any other way out to change the Inventory Unit even the transaction exists?
We must want to keep this Item as it is.

Thanks in advance...

Nadeem Zafar